This invention relates to epitaxial layers of II-VI semiconductor compounds doped with nitrogen, and more particularly relates to a method of producing such layers having improved dopant concentration and improved crystallinity.
As is known, semiconductors are characterized as either n-type or p-type, depending upon whether the predominant carriers in the material are electrons or holes. As is also known, semiconductors can be rendered n-type or p-type by substituting impurity atoms (dopants) for atoms of the host lattice which have a different valence. Donor-type impurities are those which give electrons, and thus render the host material n-type, while acceptor-type impurities are those which receive electrons, and thus render the host p-type.
Successful doping to obtain or enhance n-type or p-type conductivity depends not only on the ability to introduce a sufficient amount of the proper dopant into the semiconductor material, but also upon the ability to position the dopant atoms in the proper substitutional sites within the material's crystal lattice where they can give or receive electrons.
Dopants which do not readily assume the proper substitutional sites in sufficient number can be activated, i.e., converted to donors or acceptors, e.g., by a thermal anneal of the doped semiconductor material.
Another important consideration is the presence of other impurities in the semiconductor material which are, or are capable of assuming, an opposite conductivity type than that intended, thus compensating the effect of the dopant. Thus, it is actually the net donor or acceptor concentration which determines the overall conductivity of the material.
Semiconductors which can easily be rendered n-type or p-type, such as Si. from Group IVA of the Periodic Table, and GaAs, a III-V compound, so-called because it is made up of elements from Groups IIIA and VA of the Periodic Table, can be converted to devices such as diodes by doping adjacent regions p- and n-type to form pn junctions.
II-VI compounds such as ZnS and ZnSe are of interest for such devices because of their relatively wide band gaps. For example, being able to form a doped junction in an epitaxial layer of ZnSe could result in a blue-emitting LED or laser.
However, in practice, it has proved extremely difficult to obtain stable p-type ZnSe epitaxial layers. While a sufficient amount of dopant can usually be introduced into the layers, it is either difficult to convert sufficient numbers of the dopant atoms into acceptors, or the acceptors are unstable. For example, lithium-doped epitaxial layers of ZnSe can be converted to p-type material (defined herein as a material having a net acceptor concentration greater than 1x10**14 acceptors or holes per cc)., but lithium is unstable because of its tendency to diffuse, even at relatively low temperatures.
Nitrogen would be a more stable acceptor than lithium, and can be doped into ZnSe in situ in high concentrations (10**19/cc) using metal organic chemical vapor deposition (MOCVD). However, only a small fraction of it (up to 1.times.10**14/ cc) can be activated.
Greater success has been achieved using chemical beam epitaxy (CBE). That is, starting with an as-grown dopant concentration of about 10**19, a net acceptor concentration in the range of 10**16 to 10**17 has been achieved. However, the technique requires relatively expensive equipment and the conversion efficiency is relatively low.
A problem encountered in the MOCVD of N-doped ZnSe using NH.sub.3 as the dopant species is the limitation in the active acceptor concentration achievable due to the relative stability of NH.sub.3 at the growth temperature. NH.sub.3 is expected to decompose into NH.sub.2,NH . . . (NHx) with each subsequent species being present in decreasing concentration. Also the possibility of HxN-NHx dimer formation is likely. Increasing the decomposition of NH.sub.3 by using higher growth temperatures results in a decrease in the sticking coefficient of these species on the growth surface. Active Nitrogen acceptor is incorporated when the Hx from the NHx species which arrive at the surface is removed, possibly due to the attraction of CH.sub.3 from the metalorganic (MO) species. Attempts to increase the concentration of NHx by increasing the flow of NH.sub.3 in the growth chamber results in the degradation of crystal quality of the epi layer, probably due to NH.sub.3 reacting with the Se MO precursor.
Dopants are usually activated by a carefully controlled thermal treatment such as a furnace anneal, which allows the dopant ions to relax into the correct substitutional sites in the host lattice, and/or results in the removal of a species, such as H, which tends to passivate the dopant.
Unfortunately, such annealing, while necessary to achieve activation, often results in degradation of the epi layer, for example, by interdiffusion across the boundary surfaces of the layer.
Rapid thermal annealing has been employed in combination with a diffusion-limiting capping layer, in order to minimize degradation of the epi layer during activation. See commonly assigned copending U.S. patent application Ser. No. 851,452, filed Mar. 16, 1992.
However, it would be preferable to produce a highly doped epi layer having a greater proportion of the dopant in an active or nearly active condition, so that subsequent activation by annealing could be carried out at lower temperatures, for shorter times, or both, to achieve the same or even greater amounts of activation, with less degradation of the epi layer.